为什么我不能使用 u8 作为 Rust 数组的索引值

我是新手，我正在try 编写简单的位替换程序.

我有这个密码:

const TABLE: [u64; 8] = [
    0xC462A5B9E8D703F1,
    0x68239A5C1E47BD0F,
    0xB3582FADE174C960,
    0xC821D4F670A53E9B,
    0x7F5A816D093EB42C,
    0x5DF692CAB78143E0,
    0x8E25691CF4B0DA37,
    0x17ED05834FA69CB2,
];

fn get_part(u: u64, i: u8) -> u8 {
    ((u & (0xFu64 << (16 - i))) >> (16 - i)) as u8
}

fn process(o: u8, i1: u8, i2: u8) -> u8 {
    let left: u8 = o >> 4;
    let right: u8 = o & 0xF;
    (get_part(TABLE[left], left) << 4) + get_part(TABLE[right], right)
}

我犯了这样的错误:

error[E0277]: the trait bound `u8: std::slice::SliceIndex<[u64]>` is not satisfied
  --> src/main.rs:19:15
   |
19 |     (get_part(TABLE[left], left) << 4) + get_part(TABLE[right], right)
   |               ^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `std::slice::SliceIndex<[u64]>` is not implemented for `u8`
   = note: required because of the requirements on the impl of `std::ops::Index<u8>` for `[u64]`

error[E0277]: the trait bound `u8: std::slice::SliceIndex<[u64]>` is not satisfied
  --> src/main.rs:19:51
   |
19 |     (get_part(TABLE[left], left) << 4) + get_part(TABLE[right], right)
   |                                                   ^^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `std::slice::SliceIndex<[u64]>` is not implemented for `u8`
   = note: required because of the requirements on the impl of `std::ops::Index<u8>` for `[u64]`

我不明白为什么用u8作为索引值是非法的.如何将u8转换为兼容类型？我甚至不知道哪种类型是兼容的.